Voice coil motor feedback control circuit

ABSTRACT

A circuit for driving a voice coil motor used to position the heads of a disk drive is disclosed. The circuit consists of a an H-bridge circuit, a controller, and a feedback loop. The feedback loop prevents the BEMF from driving a voltage on the voice coil motor above the supply voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electronic circuits used to control voice coilmotors (VCM's) used in disk drives and more particularly to feedbackcircuits used to reduce the distortion in the acceleration profile ofthe voice coil motor actuated heads in a disk drive.

2. Description of the Relevant Art

The problem addressed by this invention is encountered in the headpositioning systems in the disk drive industry. Typically, a modernfixed disk drive has two or more double sided disks and over a thousandtracks per disk. Each track is divided into sectors. Each side of a diskrequires at least one head to read and write information onto thesurface of the disk. The information is usually grouped by surface,track, and sector. Consequently, an important performance characteristicof a disk drive is how quickly the heads of a disk drive can move fromone track to another, commonly referred to in the industry as head seektime. The faster the head seek time, the higher the performance of thedisk drive since the transfer rate of the information is increased ashead seek time is decreased.

It has become common in the disk drive industry to use voice coil motors(VCM's) to move the heads in a disk drive. Voice coil motors offer theadvantages of higher speed and higher track to track resolution thanstepper motors.

FIG. 1 shows a typical circuit for controlling a voice coil motor 10 asis known in the prior art. The circuit is commonly referred to as anH-configuration because the four n-channel transistors 12, 14, 16, and18 form an "H" around voice coil motor 10. It is understood in theindustry that the transistors in the H-configuration can be any commontransistor such as bipolar transistors and the like. In operation,controller 20 controls the position of the heads of a disk drive bycontrolling the current in the voice coil motor 10. For example, toaccelerate the heads in a first direction, the controller would send anenabling signal to transistor 22 turning on transistor 12 and thusapplying a high voltage to node 30. At the same time, controller 20sends a DAC signal to amplifier 34 to turn on transistors 28 and 18proportional to the strength of the DAC signal, pulling node 32 low.Since node 30 is at a higher voltage potential than node 32, currentflows from node 30 to node 32 and the heads of a disk drive will respondto the resulting magnetic field by accelerating proportionally to thecurrent magnitude. Conversely, the heads of a disk drive are acceleratedin the opposite direction by enabling transistor 26 and transistor 14,thereby raising node 32 to a higher voltage potential than node 30. Theresulting current flow will accelerate the heads in the oppositedirection.

To achieve a fast head seek time, the head is conventionally accelerateduntil it is half way to the desired track, and then decelerated untilthe head reaches its destination, as shown in FIG. 2. Thisacceleration-deceleration profile is ideally accomplished by drivingcurrent through the voice coil to accelerate the heads and thenreversing the current through the voice coil to decelerate the heads.However, during acceleration, heads store energy in the form of kineticmomentum and back electromotive force (BEMF). If the prior art H-bridgecircuit is used, the BEMF will drive the voltage on node 30 to a voltageequal to Vcc plus the BEMF which can exceed the safe operating limits ofthe circuit elements. To protect the circuit elements, the BEMF voltageis typically clamped by diodes, such as diode 13, to a safe operatingvoltage. When the BEMF voltage is clamped, it distorts the accelerationprofile as shown in FIG. 3. This distortion causes excessive wear anddrive noise which negatively affects the disk drive.

Therefore, it is an object of the invention to eliminate the distortionin the acceleration profile due to the clamping of the BEMF voltage.

It is further an object of this invention to eliminate the diodes usedto clamp the BEMF voltage.

These and other objects, features, and advantages will be apparent tothose skilled in the art from the following detailed description whenread in conjunction with the accompanying drawings and appended claims.

SUMMARY OF THE INVENTION

The invention can be summarized as a circuit for driving a voice coilmotor used to position the heads of a disk drive. The circuit consistsof a an H-bridge circuit, a controller, and a feedback loop. Thefeedback loop prevents the BEMF of the VCM from driving a voltage on thevoice coil motor above the supply voltage.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an electrical schematic of a circuit used to drive a voicecoil motor as is known in the prior art.

FIG. 2 is the desired acceleration curve profile.

FIG. 3 is the actual acceleration profile for a H-bridge circuit whichclamps the BEMF according to the prior art.

FIG. 4 is an electrical schematic according to the preferred embodimentof the circuit used to drive a voice coil motor.

FIG. 5 is a timing diagram of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A circuit for driving a voice coil motor to position the heads in a diskdrive according to an embodiment of the invention will be described.Referring now to FIG. 4, a voice coil motor 10 has a node 30 and a node32 for receiving current. Node 30 is connected to the source ofn-channel transistor 12, to the drain of n-channel transistor 14, and tothe non-inverting input of operational transconductance amplifier (OTA)50. Transistor 12 has a drain connected to a voltage supply Vcc and gateconnected to the source of n-channel transistor 22. The drain oftransistor 22 is connected to a pumped voltage Vp and the gate oftransistor 22 is connected to a controller 20. The gate of transistor 14is connected to the drain of n-channel transistor 24 and to a Vccvoltage. The gate of transistor 24 is connected to the output of an OTA36 and the source of transistor 24 is connected to a voltage reference(ground). The source of transistor 14 is connected to a sense resistor38, to the inverting inputs of OTA 36 and 34, and to the source ofn-channel transistor 18. The non-inverting input of OTA 36 is connectedto the controller 20. Transistors 12, 14, 16 and 18 form an H-bridgewherein transistors 12, 14, 16, and 18 are the first, second, third andfourth transistors of the H-bridge, respectively.

Similarly, the non-inverting input to OTA 34 is connected to controller20. The drain of n-channel transistor 16 is connected to Vcc and thesource is connected node 32. The drain to n-channel transistor 18 isconnected to node 32 and its source is connected to resistor 38. Theother end of resistor 38 is connected to ground.

Feedback loop 60 senses the voltage on node 30 and controls theconductivity of transistors 16 and 18 responsive to the voltage on node30 exceeding Vcc.

In feedback loop 60, the inverting input of OTA 50 is connected to Vccand its output is connected to node 55. The drain of p-channeltransistor 52 is connected to Vcc and the drain of p-channel transistor54. The gate of transistor 52 is connected to the gate and source oftransistor 54 and to node 55. The source of transistor 52 is connectedto drain of p-channel transistor 26 whose source is connected to thegate of transistor 16. The gate of transistor 26 is connected tocontroller 20. A current source 56 has a drain connected to node 55 anda source connected to ground. The gate of n-channel transistor 58 isconnected to node 55 and has a current path from the source oftransistor 28 to ground. Transistor 28 has a drain connected to the gateof transistor 18 and means for pulling the gate up to Vcc. The gate oftransistor 28 is connected to the output of OTA 34.

In operation, the current through voice coil motor 10 is controlled bythe controller 20 controlling H-bridge transistors 12, 14, 16, and 18. Atypical control sequence for moving the heads of a disk drive is for thecontroller to send an enabling signal to transistor 22 turning ontransistor 12 and thus applying a high voltage to node 30. At the sametime, controller 20 sends a DAC signal to amplifier 34 to turn ontransistors 28 and 18 proportional to the strength of the DAC signal,pulling node 32 low. This raises the voltage at node 30 to just belowVcc while lowering the voltage at node 32 to just above ground as shownat T₀ in FIG. 5. Consequently, current flows from node 30 to node 32thereby causing the heads to accelerate and gain velocity.

To stop the heads at a desired location, transistors 12 and 18 areturned off by controller 20 while transistors 16 and 14 are turned on bycontroller 20 as shown at T₁ in FIG. 5. At this point in the sequence,energy is stored in the VCM in the form of momentum and BEMF. Since node32 is being driven to Vcc and the VCM has a stored BEMF voltage, thevoltage at node 30 is driven to a voltage just above Vcc as shown by thevoltage spike on node 30 in FIG. 5. The BEMF above Vcc is sensed by thefeedback loop 60 which actively lowers the voltage of node 32 until theBEMF is no longer above Vcc by controlling the conductivity oftransistor 16 and transistor 18. After the BEMF voltage is absorbed bytransistor 14, the voltage at node 30 is quickly driven to just aboveground while the voltage on node 32 is driven to just below Vcc whichreverses the current flow in the VCM thereby decelerating the heads,eventually stopping the heads at the desired location as is known in theindustry.

The feedback loop 60 is comprised of OTA 50, a current mirror 53 madefrom transistors 52 and 54, a current source 56, and transistor 58. Thefeedback loop operates by sensing the voltage on node 30 and feedingcurrent into node 55 proportional to the BEMF voltage. The higher theBEMF voltage, the more current that is injected into node 55. As currentis injected into node 55 the current through the current mirror 53 isreduced which effectively reduces the gate voltage of transistor 16thereby allowing transistor 16 to operate linearly allowing node 32 todrop in voltage, as shown in FIG. 5. Additionally, the current from OTA50 is injected into the gate of transistor 58 which effectively allowsfor transistor 18 to operate linearly pulling node 32 towards ground toa voltage equal to the supply voltage minus the sensed BEMF voltageabove the supply voltage, as qualitatively shown in FIG. 5.Consequently, transistor 16 and 18 are operating in unison to drive node32 to the lower voltage necessary to keep node 30 from rising above Vcc.The BEMF voltage can then be dissipated through transistor 14. Asdiscussed above, the feedback loop 60 relinquishes control after theBEMF is dissipated.

Using the feedback circuit offers the advantages of eliminating thediodes used to clamp the BEMF voltage and the distortion in theacceleration profile due to the clamping of the BEMF voltage.

Although this invention has been described as having one feedback loop,it is understood that a second loop is necessary to handle the BEMFvoltage caused from accelerating the heads in the opposite direction.The second feedback loop would sense node 32 and control transistors 12and 14 in an analogous manner as described above. Even though theinvention has been described and illustrated with a certain degree ofparticularity, it is also understood that there a numerous methods forimplementing the feedback loop and/or the VCM control circuit withoutdeparting from the spirit of the invention, as hereinafter claimed.

We claim:
 1. A circuit for driving a voice coil motor comprising:a voice coil motor having first node and having a second node; an H-bridge circuit comprising;a first transistor having a current path between a supply voltage and the first node of said voice coil motor, and having a control element; a second transistor having a current path between the first node of said voice coil motor and a voltage reference, and having a control element; a third transistor having a current path between the supply voltage and the second node of said voice coil motor, and having a control element; and a fourth transistor having a current path between the second node of said voice coil motor and the voltage reference, and having a control element; a means for controlling said first, second, third and fourth transistors having a first output connected to the control element of said first transistor, having a second output connected to the control element of said second transistor, having a third output connected to the control element of said third transistor, and having a fourth output connected to the control element of said fourth transistor; and a feedback loop having an input coupled to the first node of said voice coil motor and having an output coupled to the control elements of said third transistor and said fourth transistor for changing the conductivity of said third and fourth transistors responsive to the voltage on the first node of said voice coil.
 2. The circuit of claim 1 wherein said feedback loop comprises an amplifier having an inverting input connected to said voltage supply and having a non-inverting input connected to the first node of said voice coil motor, and having an output for changing the conductivity to said third and fourth transistors responsive to the voltage on the first node of said voice coil motor.
 3. The circuit of claim 2 wherein said amplifier comprises an operational transconductance amplifier.
 4. The circuit of claim 2 wherein the feedback loop further comprises a first current mirror connected in series with said amplifier and the control element of said third transistor.
 5. The circuit of claim 2 wherein the feedback loop further comprises a transistor connected in series with the control element of said fourth transistor and ground.
 6. The circuit of claim 1 wherein said first, second, third, and fourth transistors are MOSFET transistors.
 7. The circuit of claim 1 wherein said H-bridge circuit, said control circuit, and feedback loop are on a single integrated circuit.
 8. A circuit for driving a voice coil motor comprising:a voice coil motor having first node and having a second node; an H-bridge circuit comprising;a first transistor having a current path between a supply voltage and the first node of said voice coil motor, and having a control element; a second transistor having a current path between the first node of said voice coil motor and a voltage reference, and having a control element; a third transistor having a current path between a supply voltage and the second node of said voice coil motor, and having a control element; and a fourth transistor having a current path between the second node of said voice coil motor and a voltage reference, and having a control element; a controller for controlling the conductivity of said first, second, third, and fourth transistors means for feeding back the voltage on the first node of said voice coil to said third transistor and to said fourth transistor.
 9. The circuit of claim 8 wherein said means for feeding back the voltage on the first node comprises an amplifier having an inverting input connected to said voltage supply and having a noninverting input connected to the first node of said voice coil motor, and having an output for changing the conductivity to said third and fourth transistors responsive to the voltage on the first node of said voice coil motor.
 10. The circuit of claim 9 wherein said amplifier comprises an operational transconductance amplifier. 